1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a multi-domain vertical alignment liquid crystal display device having a very wide viewing angle and increased transmittance while having a structure which allows the multi-domain vertical alignment liquid crystal display device to be manufactured easily and inexpensively.
2. Description of the Related Art
Generally, a vertical alignment liquid crystal display device (LCD) uses liquid crystals having negative dielectric anisotropy. When a voltage is not applied to the liquid crystal molecules, the longitudinal axis of the liquid crystal molecules are perpendicular to a surface of an alignment layer. When a voltage is applied to the liquid crystal molecules, the longitudinal axis of the liquid crystal molecules is parallel to the surface of the alignment layer.
In a mono-domain vertical alignment LCD, a gray inversion is very high at an angle of 45 degrees between a transmittance axis of a polarizer and an azimuth angle of the LCD. The high gray inversion of the mono-domain vertical alignment LCD causes the LCD to have a very small viewing angle.
To solve this problem, a multi-domain LCD was made in order to realize a wide viewing angle. Such multi-domain LCDs are described in, for example, U.S. Pat. Nos. 5,608,556 and 5,309,264.
In the conventional multi-domain LCD shown in FIG. 1, a surrounding electrode or side electrode is provided to achieve alignment of the liquid crystal molecules. FIG. 1 is a sectional view of a TFT LCD (Thin Film Transistor Liquid Crystal Display Device) having the surrounding electrode and which is manufactured by Sanyo Electric Co., Ltd., Japan.
As shown in FIG. 1, the TFT LCD includes an upper substrate 2 having an upper vertical alignment layer 3, a lower substrate 1 having a lower vertical alignment layer 3xe2x80x2, a pixel electrode 5 on the lower substrate 1, a common electrode 6 on the upper substrate 2, and liquid crystal material having negative dielectric anisotropy disposed between the upper and lower substrates 2, 1. It should be noted that the upper and lower vertical alignment layers in the structure of FIG. 1 are not alignment treated.
As seen in FIG. 1, there are slits 7 formed in the common electrode 6 on the upper substrate 2 to align the liquid crystal molecules 4 as described below.
When a voltage is not applied to the LCD in FIG. 1, the liquid crystal molecules are aligned vertically, but when a voltage having a value over a threshold voltage (Vth) is applied to the LCD, a lateral electric field (shown by curved dotted lines in FIG. 1) is generated between slit patterns 7 formed in the common electrode 6, and the longitudinal axes of the liquid crystal molecules are aligned perpendicularly relative to the lateral electric field.
In the FIG. 1, the dotted curved lines shown in the liquid crystal layer represents the lateral electric field. The slit patterns 7 formed in the common electrode 6 on the upper substrate 2 creates a boundary which is a region where the alignment directions of the liquid crystal molecules are changed from one direction to a different direction. As a result, a disclination line is formed at regions located at the slit patterns. The lateral electric field between the surrounding or side electrodes 12, 12xe2x80x2 and the pixel electrode 5 is parallel relative to the electric field between the pixel electrode 5 and slit patterns 7, and then the alignment directions of the liquid crystal molecules are determined at the corners of the pixel electrode 5.
The TFT of the conventional LCD is a top gate type, and includes a light shielding layer 13 which is arranged to prevent light from entering into an amorphous silicon (a-Si) layer 10, a source electrode 8, a drain electrode 9, and a gate electrode 11.
In this vertical alignment LCD, since a transmittance axis of a polarizer and a transmittance axis of an analyzer are crossed and perpendicular to each other, the LCD shows a black state when no voltage is applied to the LCD (Normally Black Mode). Further, each pixel is divided into four domains and the liquid crystal molecules are parallel aligned within each domain.
There are several drawbacks and disadvantages with the prior art multi-domain LCDs. Because there are disclination lines at the middle of the slit patterns or between the four domains of each pixel, the transmittance of the LCD is greatly reduced. The disclination lines of the prior art multi-domain LCDs are visible which causes the reduction in transmittance. In addition, the amount of the fringe field effect achieved is limited in conventional multi-domain LCDs so that the view angle, although wider than other mono-domain LCDs, is not sufficiently large. Further, the structure of FIG. 1 makes the multi-domain LCD very difficult to manufacture since the formation of the slit patterns 7 in the common electrode 6 is very time consuming, expensive and difficult, and requires the use of an extra mask step.
To overcome the many problems of the conventional LCD devices described above, preferred embodiments of the present invention provide an LCD which has a greatly increased view angle, much higher transmittance, and much more rapid response time than the conventional LCD devices.
More specifically, preferred embodiments of the present invention provide a multi-domain LCD having a wide-viewing angle, high brightness/transmittance and rapid response time achieved by a novel and reliable arrangement of liquid crystal molecules.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, a multi-domain liquid crystal display device includes an upper substrate including an upper alignment layer, a lower substrate including a lower alignment layer, a pixel electrode on the lower substrate, the pixel electrode being patterned to define at least one floating electrode, and a liquid crystal layer between the upper and lower alignment layers, the liquid crystal layer including liquid crystal molecules having negative dielectric anisotropy.
In another preferred embodiment of the present invention, a liquid crystal display device, includes an upper substrate including an upper alignment layer; a lower substrate including a lower alignment layer; a pixel electrode on the lower substrate, the pixel electrode being patterned to define at least one slit; and a liquid crystal layer between the upper and lower alignment layers, the liquid crystal layer including liquid crystal molecules having negative dielectric anisotropy, wherein at least one of the upper alignment layer and the lower alignment layer is alignment-treated in an alignment-treatment direction.
A further preferred embodiment of the present invention provides a liquid crystal display device including an upper substrate including an upper alignment layer, a lower substrate including a lower alignment layer, one of the upper alignment layer and the lower alignment layer being alignment-treated in an alignment-treatment direction, an analyzer, and a polarizer (provided on the upper substrate), a pixel electrode on said lower substrate, said pixel electrode being patterned to define a plurality of slits, said liquid crystal layer including liquid crystal molecules having negative dielectric anisotropy.
Another preferred embodiment of the present invention provides a liquid crystal display device including an upper substrate including an upper alignment layer, a lower substrate including a lower alignment layer, a pixel electrode on said lower substrate, said pixel electrode being patterned to define a plurality of floating electrodes and a liquid crystal layer between said upper and lower alignment layers, said liquid crystal layer including liquid crystal molecules having negative dielectric anisotropy, wherein at least a portion of the liquid crystal molecules are arranged to be twisted in at least one spiral twist pattern extending between the lower substrate and the upper substrate.
Another preferred embodiment of the present invention provides a liquid crystal display device including an upper substrate including an upper alignment layer, a lower substrate including a lower alignment layer, a pixel electrode on said lower substrate, said pixel electrode being patterned to define a plurality of slits, and a liquid crystal layer between said upper and lower alignment layers, said liquid crystal layer including liquid crystal molecules having negative dielectric anisotropy, wherein at least a portion of the liquid crystal molecules are arranged to be twisted in at least one spiral twist pattern extending between the lower substrate and the upper substrate.
In one preferred embodiment of the present invention, an angle between a longitudinal axis of the slits or floating electrodes and the alignment direction is preferably equal to or less than about 50 degrees.
A value of xcex94n (Optical anisotropy of the liquid crystal molecule in light wavelength of 550 nm)xc3x97dLC (Thickness of the liquid crystal layer) is preferably about 0.25 xcexcm to about 0.45 xcexcm.
A preferred embodiment of the liquid crystal display device preferably includes a compensation film, wherein (((xcex7x+xcex7y)/2)xe2x88x92xcex7z)xc3x97dc is preferably about 0.8 to about 1.2 times xcex94ndLC.
(((xcex7x+xcex7y)/2)xe2x88x92xcex7z) is the difference between the average refractive indexes of lights vibrating in a horizontal direction and a vertical direction of the compensation film.
dc: Thickness of the compensation film
xcex94n: Optical anisotropy of the liquid crystal molecule
dLC: Thickness of the liquid crystal layer.
A pretilt angle of the upper alignment layer is preferably about 84 to about 89 degrees.
A width of a short axis of the slit or floating electrode is preferably about 5 xcexcm to about 15 xcexcm.
A preferred embodiment of the liquid crystal display device further includes a polarizer, wherein an angle between a longitudinal axis of the at least one slit or floating electrode and a transmittance axis of the polarizer is about 0 degree to about 50 degrees.
The liquid crystal molecules between the adjacent slits or floating electrodes have a left-handed twist arrangement or a right-handed twist arrangement.
The upper alignment layer is preferably rubbing-alignment-treated or photo-alignment-treated.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.